1. Field of the Invention
The invention relates generally to acoustic well logging. More particularly, the invention relates to methods and systems for acoustic log data processing.
2. Background Art
The oil and gas industry uses various tools to probe the formations penetrated by a borehole in order to locate hydrocarbon reservoirs and to determine the types and quantities of the hydrocarbons. Among these tools, acoustic tools have been found to provide valuable information regarding formation properties. In acoustic logging, a tool is typically lowered into a borehole, either after the well is drilled or while the well is being drilled, and acoustic energy is transmitted from a source into the borehole and the formation. The acoustic waves that travel in the formation are then detected with an array of receivers.
Modern acoustic tools generally have multipole sources. The multipole sources may include one or more monopoles, dipoles, and quadrupoles to excite different modes. Monopole mode excitation is traditionally used to generate compressional and shear head waves. From the monopole measurements, formation compressional and shear slowness can be obtained by processing the head wave components. However, in slow formations, which are defined as having shear slowness higher than the borehole fluid slowness, the shear head waves are not measurable. Therefore, in slow formations, shear wave logging has to rely on borehole modes, such as dipole modes for wireline tools or quadrupole modes for logging-while-drilling (LWD) tools, to indirectly provide the formation shear slowness.
Unlike monopole head waves, the dipole or quadrupole borehole modes are dispersive. The characteristics of the dipole or quadrupole modes depend on formation shear slowness (DTs) as well as many other borehole-formation parameters, such as formation compressional slowness (DTc), formation density (ρb), mud slowness (DTm), mud density (ρm), and hole diameter (HD). Thus, in order to derive formation shear slowness (DTs) from the dipole or quadrupole modes, knowledge of these other borehole-formation parameters is needed.
One known method for processing the dipole or quadrupole dispersive wave components to obtain the shear slowness is the dispersive slowness-time-coherence (DSTC) method disclosed in U.S. Pat. No. 5,278,805 issued to Kimball (assigned to the present assignee and incorporated herein by reference in its entirety). See also, Kimball, Geophysics, Vol. 63, No. 2, March-April 1998. The DSTC method assumes all borehole-formation parameters, except DTs, are known. These borehole-formation parameters may be obtained from other logging operations or from the known dimensions of the borehole and the tool.
The accuracy of the DSTC processing depends on the accuracy of the other assumed known borehole-formation parameters: DTc, ρb, DTm, ρm, and HD. In other words, any inaccuracy in these other borehole-formation parameters may have an impact on the accuracy of the derived formation shear slowness (DTs). Sensitivity analysis indicates that mud slowness (DTm) has great impact on the accuracy of formation shear slowness (DTs) derived from DSTC processing. The influence of DTm is particularly significant in fast rocks such that without an accurate mud slowness, it is very difficult, if not impossible, to obtain an accurate formation shear slowness (DTs) with DSTC processing. This is true even if high quality measured waveforms, with the best possible signal-to-noise ratio, are available. However, accurate DTm values are not always available. Therefore, a need remains for more robust techniques for shear slowness estimation. In particular, techniques that can produce DTs without prior knowledge of other borehole-formation parameters are highly desirable.